1. Field of the Invention
The invention relates to a sheet feeder, and more particularly to a sheet feeder capable of immediately detecting a paper jam.
2. Description of the Related Art
Recently, a scanning device has become one of the widely used products in the office or personal application. Documents may be digitized by the scanning device, and also may be copied by the scanning device in conjunction with an output device such as a printer.
With the increase of the number of to-be-scanned documents, the conventional flat-bed scanner cannot be used to quickly scan the documents. Hence, a sheet-fed scanner and a flat-bed scanner in conjunction with a sheet feeder have become important products to meet the requirements.
A paper jam may sometimes occur when the sheet feeder feeds documents. The mostly used technology for detecting a paper jam in the sheet feeder is to cause the jammed paper to trigger some mechanism or sensor during the feeding procedure and then to inform the sheet feeder to stop.
In the above-mentioned technology, however, when the sheet feeder stops feeding the document, the jammed sheet may be damaged and cannot be well preserved.
Alternatively, it is possible to detect a paper jam by counting the time for a sheet to be fed to a predetermined position. Conventionally, a sheet detector may be provided in the sheet passage of the sheet feeder to detect whether or not the sheet has reached the position of the sheet detector in a predetermined period of time. If the sheet does not reach the detector in the predetermined period of time, it is judged that a paper jam has occurred.
According to the above-mentioned technology, a relatively long period of time has to be considered in order to completely and precisely judge a paper jam. For example, it is judged that a paper jam occurs if the sheet does not reach the sheet detector in five seconds after the sheet is fed. Because it is necessary to consider the manufacturing errors and safety factors, the judgement time may not be effectively shortened.
Consequently, it is an important subject of the invention to provide a sheet feeder capable of immediately detecting a paper jam.